In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the wetted printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
Current laser-based lithographic systems frequently rely on removal of an energy-absorbing layer from the lithographic plate to create an image. Exposure to laser radiation may, for example, cause ablation—i.e., catastrophic overheating—of the ablated layer in order to facilitate its removal. Because ablation produces airborne debris, ablation-type plates must be designed with imaging byproducts in mind; for example, the plate may be designed so as to trap ablation debris between layers, at least one of which is not removed until after imaging is complete.
Alternatives to ablation-type plates include plates utilizing a less energetic imaging mechanism, such as polymer fusion or coalescence. For example, polymer particles may be dispersed within a water-soluble binder, which holds the particles together; the binder is applied as a coating onto a substrate. The coating also contains a material that absorbs imaging (e.g., infrared or “IR”) radiation and converts it to heat, which fuses the particles into a monolithic solid feature. In use, the plate is exposed to imaging radiation in an imagewise fashion, selectively fusing the particles. The unexposed regions of the particle-containing coating are then washed away, typically using a processing solution with mild enough properties to be considered a post-development gum.
Unfortunately, these plates can be difficult to handle and store. The binder's water solubility facilitates post-imaging development, but also imparts sensitivity to fingerprints or handling damage—particularly in wet or humid environments.